The invention relates to a polder principle comprising upright shielding walls which are located opposite one another and are inserted into the ground relatively wide apart and which are connected to one another at the bottom in an essentially water-impermeable manner. The invention also relates to a method for implementing said polder principle.

In regions where the groundwater level is relatively high it is known to apply the polder principle for making watertight areas below ground level. To this end a surface region extending to below the groundwater table is provided with a watertight shield using so-called dam walls. Care being taken that the bottom of the dam walls project into an essentially horizontal, watertight layer located below the groundwater table. The watertight layer can be a naturally occurring barrier layer composed of, for example, clay. Said watertight layer can also be a concrete floor or a watertight sand layer obtained by injection of, for example, water-glass or other in ectable materials. The advantage in using a naturally occurring barrier layer is that a watertight polder principle is obtained immediately on inserting the dam wall into the ground, as a result of which excavation work can be carried out inside the polder principle without problems due to the ingress of groundwater from the surrounding area and the associated need for (temporary) well drainage with the associated adverse lowering of the groundwater level. If it is to offer adequate resistance to the groundwater pressure, a polder which has a naturally occurring watertight floor can usually be only partially excavated.

If said natural barrier layer is absent, it is necessary, after inserting the dam wall, to dig out the earth inside down to the level where the watertight floor has to be installed. This excavation work is usually carried out in the wet, because groundwater from the surrounding area continues to flow in freely via the underside. Lowering of the surrounding groundwater level can be used to restrict said flow. After the watertight floor has been installed, for example by pouring underwater concrete, the polder created can be emptied by pumping out. If the watertight floor is insufficiently heavy, ballast must be placed on top and/or tension posts or anchors connected to the floor are needed to offer adequate resistance to the upwards pressure of the groundwater.

Excavation work and well drainage are not necessary if use is made of

injection to produce the watertight floor. However, the injection of large areas is expensive.

In view of the fact that the free space between the shielding walls of the known polder principle is essentially the same at the top and the bottom, a watertight floor of relatively large surface area is required if the area below ground has a relatively large floor area.

The aim of the invention is to provide an alternative polder principle and method for the implementation thereof, which in the case of the absence of a naturally occurring watertight floor is more effective than the known polders created using watertight walls and floors and described above.

To this end the shielding walls inserted in the ground opposite one another delimit a funnel-shaped region narrowed towards the bottom. Thus, during the production of the polder principle the shielding walls are inserted into the ground sloping towards one another, as a result of which the gap between them at the bottom becomes appreciably smaller than that at ground surface level. For example, the angle of slope of the shielding walls is more than 10° with respect to the vertical. Consequently, underground a watertight seal between the shielding walls located opposite one another has to be provided only in a relatively small region, whist retaining a large floor surface area for the area below ground. This can be effected by injecting, for example, water-glass into the gap at the bottom of the shielding walls, but also by excavating the area enclosed by the shielding walls and pouring, for example, underwater concrete to seal the relatively small gap at the bottom of said area. Injection is to be preferred because no excavation is required to create the polder principle. However, in comparison with the conventional polder principles having a watertight floor produced in the wet, as a consequence of the small gap between the shielding walls less trouble from welling-up groundwater is experienced when excavation is carried out down to the bottom of the shielding walls. Anchoring of the watertight floor is also more secure, since ,said floor presses from below against the sloping shielding walls, which because of their inclined position are better able to absorb the upwards forces exerted by the watertight floor.

The invention also offers the possibility of transmitting foundation forces via the sloping shielding walls. In the case of the known polder principles separate anchors or other foundation means are used for this purpose. Because the shielding walls located opposite one another are relatively close together at the bottom, a relatively rigid connection

between said shielding walls is formed at this location. As a result the risk of deformation of the polder construction is reduced and more passive ground pressure action can be mobilised. For example, the enclosed area can be excavated to greater depth as a result. Moreover, the shielding walls positioned in a funnel-like manner exert a more reliable ground-retaining and ground-stabilising action.

Furthermore, it is, of course, also possible to make use of a naturally occurring watertight layer. In such a case the invention is, for example, applicable if the quality of said naturally occurring watertight floor is reliable only within a small region. If there is a naturally occurring watertight layer but this is at too great a depth, it is nevertheless possible, using the invention, to create an inexpensive polder principle, which does not have to extend as far as said naturally occurring watertight layer. It is known per se to insert shielding walls into the ground slightly sloping at an angle of no more than 5°- The aim in this case is to reduce the lateral pressure exerted by the ground on the shielding walls. The aim of the known positioning at a slight slope is not to insert shielding walls in the ground opposite to and sloping towards one another so that a region which is narrowed towards the bottom is confined, only a small region at the bottom thereof having to be made watertight. In addition, it is also known to dredge a funnel-shaped trench in the ground, after which the walls thereof are provided with a watertight lining using sheeting, which is stabilised with ballast, and the delimited area is then pumped dry, for example in order to produce a sunken road. The aim in this case is not to install the watertight lining without extensive excavation and to keep the excavated hole dry during work to create the polder principle.

Suitable shielding walls are all conventional bend-resistant shielding walls, such as steel dam wall plates, or soft shielding walls, such as sheeting screens. The length thereof is usually several tens of metres. Said walls are vibrated, pressed or driven directly into the ground or inserted in a narrow trench filled with bentonite or the like.

Although the invention has been discussed with reference to preventing groundwater welling up in areas below ground, the invention can also be used in other terrains.

Other advantages and details of the invention will become apparent from the following description of two non-limiting embodiments with reference to the drawings. In the drawings:

Fig. 1 shows a diagrammatic cross-sectional view of a polder principle according to the invention, used for a sunken railway; and

Fig. 2 shows a view, corresponding to Fig. 1, of an alternative. Fig. 1 shows a trench 1 which has been made in the ground 2 and on the bottom of which two rail tracks 3 have been laid for the purposes of rail traffic 4. The tracks 3 are below the groundwater table 5. The trench is covered by a roof, shown as a conceptual roof 6, over which earth removed during excavation of the trench is tipped in a layer shown as a conceptual layer 7- Support pillars 8 are also shown conceptually. Before excavating the trench 1, conventional steel dam wall plates 9 are vibrated into the ground in the conventional manner with the longitudinal sides linked to one another to provide a seal. According to the invention, to this end plates 9 are introduced into the ground 2 opposite one another some distance apart and sloping sharply towards one another, as a result of which a region which in the drawing is narrowed towards the bottom and is essentially V-shaped is delimited. The plates 9 leave a small gap at the bottom, which gap is sealed watertight using a plug 10. Said plug 10 is produced by injecting water-glass via an injection tube which runs down one plate 9 and through which a proximity switch was initially lowered (not shown) . It can be seen that in the drawing the left plate 9 extends further downwards than the right plate 9- By this means it is advantageously ensured that the gap which is left at the bottom between the plates 9 is reliably and sufficiently small. At the plug 10 the gap is about one metre. At ground level the gap is about fifteen metres. The plates 9 extend about twenty metres deep.

Using the invention it is possible, in a particularly advantageous manner, to produce a watertight gully which is sunken to below the groundwater table and extends over a great length in a completely continuous process. Fig. 2 shows an embodiment using flexible sealing shielding walls. In order to protect the flexible sheeting 11 against the lateral ground and water pressure, the trench 1 is excavated in such a way that a bank 12 remains on either side. This technique can, of course, also be used for more bend-resistant shielding walls. Other variants of the invention are, of course, also possible. The important feature is that the dam wall plates are introduced into the ground opposite one another in such a way that they are relatively wide apart at the top and relatively close together at the bottom.